Metal powder from cast iron chips



F b- 1 1968 w. SCHROEDER ETAL 3,368,890

METAL POWDER FROM CAST IRON CHIPS Filed Dec. 27, 1966 CAST IRONPARTICLES SCREEN PULVERIZE OVERSIZE CLASSIFY IRON OXIDE ADDITION ANNEALSEPARATION OF FINES POWDER SUITABLE FOR POWDER METALLURICAL TECHNIQUESINVENTORS ezwiz m TORNEY United States Patent 3,368,890 METAL POWDERFROM CAST IRON CHIPS William L. Schroeder and Peter Vernia, Rochester,Mich, assignors to General Motors Corporation, Detroit, Mich, acorporation of Delaware Filed Dec. 27, 1966, Ser. No. 605,064 7 Claims.(Cl. 75-211) ABSTRACT OF THE DKSULQSURE Iron powders particularlysuitable for compaction by powder metallurgical techniques may beprepared from cast iron particles such as chips, borings, or the like.The particles are first comminuted to a powder of suitable particlesize. An iron oxide is then blended with the powder and the mixtureannealed at an elevated temperature below the austenite region in areducing atmosphere. After cooling, any loose graphite or ultrafine ironpowder is separated from the mixture.

This invention relates to a method of preparing iron powder for powdermetallurgical processing. More particularly the invention relates to thepreparation of powdered iron from cast iron chips, borings, turnings,and the like.

Many articles of commerce are prepared by compacting metal powders underhigh pressures into a compact of desired configuration and subsequentlysintering the compact. The physical strength of the article so producedand the shrinkage or expansion of the compact upon sintering are afunction of the properties of the starting material. In this regard,sponge iron powders are known to be particularly suitable for ferrouspowder metallurgical operations. Ferrous powder in sponge form may bepressed to form a compact having a relatively high green fiber strengthand a relatively low distortion upon sintering. To be more specific,sponge iron mixed with 1-25 graphite may be pressed at 30 tons persquare inch into a compact having a green fiber strength in excess of1000 p.s.i. Moreover, when the compact is sintered the change indimension is very slight, there being an expansion of about 0.1 to 0.2%.While these are very suitable properties in a compact, sponge iron ismore expensive than powder produced in other forms. It would bedesirable for instance to produce a powder having compaction propertiessimilar to sponge iron which is not as expensive.

Accordingly, it is an object of this invention to provide a method ofproducing iron powder, suitable for compaction by powder metallurgicaltechniques, from cast iron particles.

It is a more specific object of this invention to provide a method ofconverting cast iron chips, borings, and the like to a powder which issuitable to be compacted into an article having high fiber strengthbefore sintering and low shrinkage upon sintering.

There and other objects are accomplished in accordance with ourinvention by first comminuting cast iron borings, chips, or the like toa powder of suitable particle size, preferably smaller than about 90mesh. To the iron powder is added oxygen in the form of an iron oxide ina small but effective amount preferably comprising about 0.05 to 0.6% ofthe weight of the iron powder. The mixture is then heated at an elevatedtemperature below the critical temperature to react the iron oxide withthe uncombined carbon (graphite) which coats the iron powder aftercomminuting. At the same time that the iron oxide is reacting withgraphite on the surface of the iron powder the heat is effecting somechanges in the pearlitic cementite within the iron powder. A portion ofthe pearlitic iron carbide is decomposed into graphite and iron. Much ofthe rest of the iron carbide is partially spheroidized. The net effectof these changes within the iron powder is to render it more ductile.The heating is accomplished in a reducing atmosphere. While the powderis still in the reducing atmosphere it is cooled to about roomtemperature and any remaining loose graphite together with ultrafineparticles of iron are removed from the mixture to reduce the totalcarbon content preferably to about 2% by weight. This powder is thenready to be compacted and sintered into a useful article of commerce. Inan alternative embodiment of the invention the iron powder so producedmay advantageously be blended with up to about 30% by weight sponge ironprior to compaction.

Other objects and advantages of our invention will become apparent froma detailed description of the method which follows. During thisdescription reference will be made to the drawing which is a blockdiagram of a preferred embodiment of the process illustrating theprinciples of the invention.

Referring to the drawing, the feed stock comprises cast iron particles,which may be of scrap origin such as chips, borings, and the like solong as they are substantially free of oil and moisture. Depending uponthe nature and capacity of the com-minuting equipment employed it may beadvantageous to screen or otherwise classify the starting material forefiicient operation. The cast iron particles are comminuted preferablyby high impact pulverization means, such as a hammer mill or the like,to a powder of particle size which is suitable for powder metallurgicalprocessing. In general, it is preferred that about 95% of the powderwill pass a mesh Tyler standard screen and about 23-24% of the powderwill pass through a 325 mesh Tyler standard screen. The comminutingoperation may be conveniently performed in commercially availableequipment such as a hammer mill. For particularly efficient operationthe comminuting equipment can be employed in combination with aninternal centrifugal classifier whereby the fine -90 mesh powder isdischarged from the classifier and coarser material is recirculateduntil it has been broken down to a powder of suitable dimensions. Alsoin connection with the comminution step, a cyclone separator can beoperated in combination with the pulverizer to remove excessively fineiron, e.g. 10 to 20 microns in diameter, and free graphite particlesfrom the 90 mesh powdered iron.

In prior art techniques cast iron powders so produced have subsequentlybeen annealed to relieve the stresses induced by 'pulverizatidn and thencompacted into articles of commerce. However, upon sintering, thesecompacts have undergone excessive shrinkage. Moreover, the fiberstrength of the unsintered article is too low for many applications.Accordingly, we have developed a process by which the comminuted powdermay be treated to produce a powder which is particularly useful forpressing into a compact which has minimum dimension change uponsintering and has a markedly improved fiber strength.

In accordance with the invention, a small amount of oxygen in the formof an iron oxide is blended with the comminuted cast iron powder. Somebenefits are obtained from very small additions. However, best resultsare obtained when an amount of oxygen equivalent to 0.05 to 0.6% of theweight of the iron powder is added. In general, this can be accomplishedby adding to 2% of an iron oxide such as FeO, R2 0 or Pe o, based uponthe weight of the cast iron powder treated. The iron oxide is employedin ultrafine powder form. Preferably the greatest dimension of anaverage particle is ten microns or less. Iron oxide particles of thissize will coat the particles of iron to more effectively react with thegraphite thereon.

Subsequently, this powdered mixture is heated in a suit able furnacecontaining a neutral or slightly reducing atmosphere to an elevatedtemperature below the austenitic transformation temperature of the castiron. Preferably a temperature in the range of 1000 F. to 1400 F. isemployed. These temperatures are commonly known as annealingtemperatures in the art offerrous metallurgy. The effect and purpose ofthe heating or anneal is many fold. As in the prior art one effect is torelieve the stresses induced in the powder by the comminuting step. Inaddition, however, the iron oxides incorporated upon the pulverized castiron react rapidly with the free graphite which is deposited upon theiron powder in a thin film during the comminution step. Carbon monoxideis evolved and the concentration of graphite on the powder is decreasedwhile the iron oxide is reduced to elemental iron. At the same timeduring the anneal, the pearlitic carbides within the iron powder, aregraphitized or partially spheroidized to render the iron more ductile.

We have found that particularly beneficial results are obtained if theheating or anneal is conducted sequentially at two differenttemperatures. First, the powders are heated to about 1050 F. At or belowthis temperature, Fe O or Fe O is believed to be directly converted toiron and carbon monoxide without forming a lower oxide. This reactionpath may reduce the required treatment time of the iron oxides if theseparticular oxides are used. Moreover, it is believed that by treatingthe powder at a lower temperature before heating further to a highersubcritical temperature, nuclei are formed which enhance thegraphitization and spheroidization of cementite. Thus, it is preferredthat the powder be heated at 1050 F. for about two hours and thenimmediately raised to about l400 for an additional two hours. Theaddition of iron oxide to the comminuted powder has had the effect ofmarkedly increasing the green fiber strength of cast iron powders sotreated. For example, a compact formed by compressing milled andannealed cast iron borings under a pressure of 30 tons per square inchwas found to have a green fiber strength of 250 p.s.i. However, acompact formed from powders prepared in exactly the same manner, exceptthat an addition of 0.5% by weight Fe O was made prior to annealing, wasfound to have a green fiber strength of over 400 p.s.i.

Preferably the atmosphere of the furnace, in which the annealing orheating step is carried out, is neutral or slightly reducing. An exampleof a suitable atmosphere is one comprised of 95% nitrogen and 5%hydrogen. Atmospheres employing up to 98% nitrogen and about 2% hydrogenhave been successfully used, but the higher hydrogen content ispreferred. The annealing treatment may be prolonged beyond the total offour hours specified above to obtain further spheroidization ofpearlitic cementite but little or no additional improvement in thephysical properties of the compacted articles is obtained from powder sotreated.

After annealing the iron powder is cooled, preferably w1thin thereducing atmosphere, to prevent oxidation. Upon cooling to a suitabletemperature at or near room temperature, remaining loose graphite andultrafine iron particles (l020 microns in greatest dimension) areseparated from the mixture. This may be readily accomplished, forexample, in a counter flow type air separator wherein the powders aredirected downwardly against an upwardly flowing stream of air. The lightgraphite particles and the fine iron particles are carried upwardly withthe air out of the separator while iron powder of desired dimensiondrops to the bottom. The product withdrawn from the bottom of theseparator is a ductile iron powder which contains carbon preferably inu'ncornbined form which has been at least partially spheroidized so asto increase the toughness of the material. Powder at this stage of theprocessing typically contains only about l.8%2% by weight carbon. It maybe directly compacted in a suitable press under a pressure of about 30ton p.s.i. into a desired configuration. The green fiber strength of thecompact so produced is typically about 400 p.s.i. However, the compactis then sintered for 30- 50 minutes at 2040-2090 F. whereupon an articlehaving a minimum fiber strength of 60,000 pounds p.s.i. is obtained. Ithas been observed that the linear shrinkage upon sintering of thecompact so produced is about 1.3% or less.

If linear shrinkage of about 1.3% upon sintering cannot be tolerated ithas been found that up to about 30% by weight sponge iron mayadvantageously be mixed with the powdered cast iron prior to compacting.Sponge iron powders are known to effect an increase in the size ofpowder compacts upon sintering. Thus, the addition of the more expensivesponge iron will increase the linear shrinkage upon sintering. It willalso tend to increase the green fiber strength of the compact. We havefound that a mixture of 70 parts of cast iron powder produced inaccordance with our invention, 30 parts sponge iron, and about 0.3 partgraphite (to compensate for the absence of carbon in the sponge iron)may be compressed at 30 tons per square inch to yield a compact having agreen fiber strength of about 700 p.s.i. Upon sintering at 2040 F.2090F. for 30 to 50 minutes a minimum fiber strength of 60,000 p.s.i. isobtained and the linear shrinkage is only about 0.3%.

A specific example of the invention would be useful for purposes ofillustration. Cast iron borings comprising by weight about 3.4% carbon,2.3% silicon, 0.2% phosphorus, 0.08% sulfur, and the balance iron werepulverized in a commercially available hammer mill which contained aninternal centrifugal classifier. The classifier was adapted to passparticles mesh and smaller, the oversize particles being immediatelyrecycled. A cyclone separator (air classifier) was employed on the -90mesh powder to remove excessively fine iron and free graphite particles.In this manner sufficient graphite was removed to lower the carboncontent from 3.4% to about 2.6%.

A small amount of Fe O powder, comprising about 0.5% by weight of thecast iron powder, was mixed with the powdered iron. The mixture wasannealed two hours at 1050 F. and then two hours at 1400 F. Theatmosphere in the furnace comprised nitrogen and 5% hydrogen. Theannealed powder was cooled to about normal room temperature.

The ultra fine iron, 10 to 20 microns in greatest dimension, and theloose graphite were removed from iron powder by counter flow airseparation. This was accomplished by charging the powder at the top of avertically disposed tube downwardly against an upwardly flowing streamof air. The fine iron and graphite were carried out with the air. About45% by weight of the material was lost and carbon content was reducedfrom 2.6% by weight to about 2.0% by weight.

A portion of the powdered product of the separation process was pressedunder a pressure of 30 tons per square inch. The resulting compacts hada green fiber strength of 400 p.s.i. These compacts were sintered at2080 F. and 40 minutes. The fiber stress of the sintered compacts was inexcess of 60,000 p.s.i. and the linear shrinkage upon sintering was1.3%.

A second portion of the powdered iron as prepared above was mixed with30% of its weight of sponge iron and 0.3% of its weight of graphite.This mixture was compressed under 30 tons per square inch to yield acompact having a green fiber strength of 700 p.s.i. This compact wassintered as above to yield a structure having a fiber strength in excessof 60,000 p.s.i. with a shrinkage of 0.6%.

While our invention has been described in terms of a preferredembodiment, it is, of course, realized that other forms may be readilyadapted by those skilled in the art. Therefore, our invention is to beconsidered limited only by the scope of the appended claims.

We claim:

1. A method of producing particulate iron suitable for powdermetallurgical processing from cast iron particles comprising the stepsof comminuting clean, dry, cast iron particles to a fine iron powder,the particles of said iron powder having on the surface thereof loosegraphite as a consequence of said comminuting step; mixing oxygen in theform of an iron oxide with said iron powder, said iron oxide being inthe form of particles which are substantially smaller than said castiron powder particles so as to coat the surface thereof and the amountof said oxide being small but sufficient to react with said loosegraphite coating on said powdered iron particles; heating said mixtureto an elevated temperature below the austenitic transformationtemperature of the cast iron powder for a time whereby said iron oxideis substantially fully reacted with said loose graphite coating andwhere- 'by the iron carbides which are inherently present as pearliticcementite in said cast iron are in part graphitizer and the restpartially spheroidized; cooling said reacted mixture and separatingtherefrom loose graphite and ultrafine iron to yield said particulateiron for powder metallurgical processing.

2. A method as in claim 1 wherein the amount of said iron oxide which isblended with said iron powder is sufiicient to add 0.05%0.6% by weightof oxygen based upon the weight of said iron powder.

3. A method of producing particulate iron suitable for powdermetallurgical processing from cast iron particles comprising the stepsof comminuting clean, dry, cast iron particles until -at least about 95%by weight of said particles will pass through a 90 mesh screen, theparticles of said comminuted cast iron having on the surface thereofloose graphite as a consequence of said comminuting step; mixing smallamounts up to about 2% by weight of an iron oxide with said iron powder,said iron oxide being in the form of particles which are substantiallysmaller than the comminuted cast iron particles so as to coat thesurface thereof; heating said mixture in a reducing atmosphere to anelevated temperature below the austenitic transformation temperature ofsaid mixture for a time whereby said iron oxide is substantially fullyreacted with said loose graphite coating and whereby the iron carbideswhich 'are inherently present as pearlitic cementite in said cast ironare in part graphitized and the rest partially spheroidized to rendersaid comminuted iron more ductile; cooling said reacted mixture in a reducing atmosphere to about room temperature and separating loosegraphite and iron particles smaller than -20 microns in largestdimension from said reacted mixture to yield said particulate iron forpowder metallurgical processing.

4. A method as in claim 3 wherein said blended mixture of said cast ironand iron oxide is heated at a tema 6 perature in the range of about 1000F. to 1400 F. for a period of up to about four hours.

5. A method of producing particulate iron suitable for powdermetallurgical processing from cast iron particles comprising the stepsof comminuting clean, dry, cast iron particles to a fine iron powder,the particles of said iron powder having on the surface thereof loosegraphite as a consequence of said comminuting step; mixing oxygen in theform of an iron oxide with said iron powder, the amount of oxygen being(MOS-06% by weight of said iron powder and said iron oxide being in theform of particles which are substantially smaller than said cast ironpowder particles so as to coat the surface thereof; heating said mixtureto an elevated temperature below the austenitic transformationtemperaure of said mixture for a time suflicient to react substantiallyall of said iron oxide with said loose graphite and whereby the ironcarbides which are inherently present as pearlitic cementite in saidcast iron are in part graphitized and the rest partially spheroidized';cooling said reacted mixture; separating loose graphite and ultrafineiron from said reacted mixture; and mixing up to 30% by weight of spongeiron powder with the remaining iron powder to yield a particulate ironsuitable for powder metallurgical processing.

6. A method of producing an article of manufacture wherein the ironpowder produced in accordance with the method of claim 1 is compressedunder pressure of at least 25 tons p.s.i. into a compact ofpredetermined configuration and said compact is sintered at atemperature of 20402090 F. for 3050 minutes.

7. A method of producing an article of manufacture comprising the stepsof compacting the powdered iron and sponge iron mixture produced inaccordance with claim 5 into a compact of predetermined configurationunder a pressure of at least 25 tons per square inch and subsequentlysintering said compact at a temperature in the range of 20402090 F. fora period of 30-50 minutes.

References Cited UNITED STATES PATENTS 2,541,153 2/1951 Chadwick .55 X2,775,516 12/1956 Shafer 75.54 2,784,073 3/1957 Michalke 75-.553,073,695 1/1963 Silbereisen 75-.55 3,194,658 7/1965 Storchheim 75-211 X3,326,676 6/1967 Riibel 75-201 CARL D. QUARFORTH, Primary Examiner.BENJAMIN R. PADGETT, Examiner. A. J. STEINER, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,368,890 February 13, 1968 William L. Schroeder et a1.

pat-

d that error appears in the above numbered It is hereby certifie andthat the said Letters Patent should read as ent requiring correctioncorrected below.

Column 1 line 58 for "There" read These column 4 line 13, for "increase"read decrease column 5, line 17, for "graphitizer" read graphitizedSigned and sealed this 15th day of April 1969.

(SEAL) Attest:

EDWARD J BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

